Nuclear magnetic resonance (NMR) resonance spectroscopy and magnetic resonance (MR) imaging require an external static magnetic field to polarize the magnet moment of nuclei. The quality of the static magnetic field is gauged by its strength, spatial homogeneity, and temporal stability. Low-temperature superconductors (LTS) operating in the self-persistent mode represent the state-of-the-art technology for generating high quality magnetic fields. Persistent mode magnets provide spatially homogeneous fields with small temporal field fluctuations over a large period of time via the aid of shims and a field-frequency lock (FFL). However, the maximum field strength of self-persistent magnets is limited to 23.5 Tesla by the physical properties of LTS materials. Furthermore, to maintain a superconducting state, the LTS materials must be cooled to the temperature of liquid helium.
Increasing the field strength beyond 23.5 Tesla offers unique opportunities for studying NMR phenomena by increasing the chemical shift resolution and signal-to-noise ratio (SNR), while decreasing quadrupolar line broadening in solids. Higher fields also increase the spatial resolution of MR images and permit physiologic MR studies using low-gamma nuclei that require larger field strength to maintain measurement SNR. Eliminating the need for liquid helium cooling enables the development of compact systems that will extend the use of MR systems from hospitals to physician's offices.
Powered magnets provide fields strengths substantially greater than 23.5 Tesla and some designs do not require liquid helium cooling. In contrast to persistent mode magnets, powered magnets, such as resistive or hybrid magnets, require an external power source. Resistive magnets use a normal-metal coil. Hybrid magnets may use a combination of normal metals and superconductors. For example, powered magnets that surround an inner high-temperature superconductors (HTS) coil with an outer LTS coil are currently being developed. These LTS/HTS magnets have produced field strengths up to 24 T.
Another example of powered hybrid magnets combines normal and LTS coils to produce fields significantly greater than 24 T. One such magnet produces fields up to 45 T, i.e. almost twice what is available with current LTS/HTS magnets. The resistive and LTS coils in such a magnet are connected to separate power supplies to control the operating current of each coil individually.
The National High Magnetic Field Laboratory (NHMFL) is constructing a hybrid magnet designed to produce fields up to 36 T. While the NHMFL hybrid magnet will not have as high a field as the world record 45 T hybrid magnet, the NHMFL powered magnet is designed for higher spatial field homogeneity with less temporal field fluctuations. Unlike the 45 T hybrid magnet, the resistive and LTS coils of the NHMFL magnet will be operated using the same current and thus allows the resistive and LTS coils to be connected in series using the same power supply. The series mode operation will increase the inductance from tens of mH to hundreds of mH, intrinsically reducing temporal field fluctuations. Additionally, the single power supply reduces operating cost and the 40 mm bore is appropriate for NMR probes.
In spite of the advantages of powered magnets, there are still significant challenges for performing NMR spectroscopy and MR imaging using this technology. For example, the Series-Connected Hybrid (SCH) magnet will produce fields that are less spatially homogeneous and thereby broaden the linewidth and reduce the signal-to-noise ratio (SNR) of the MR signal. Sample spinning and field shimming improve spatial field homogeneity.
Furthermore, despite the increased inductance of the SCH magnet, this magnet will be subject to temporal field fluctuations from power supply ripple and variations in cooling water temperature and flow rate. Higher frequency fluctuations due to power supply ripple broaden the MR spectra linewidth. Also, lower frequency fluctuations due to cooling water variations distort the amplitude and phase of consecutive MR signals by causing the peaks of consecutive MR spectra to shift in frequency. Field fluctuations in uncompensated powered magnets are on the order of 10 ppm. It is appreciated that both the linewidth broadening and MR spectra peak shifts adversely affect MR studies. In particular, temporal fluctuations preclude most NMR spectroscopy and MR imaging studies relying on phase-coherent averaging of consecutive magnetic resonance signals, for example, two dimensional NMR. Therefore, in order to use the SCH magnet, or other powered magnets, for MR, field fluctuations must be significantly reduced in the 0-1 kHz frequency range to 0.01 ppm or less.
Given the above, an improved system and method for compensating or reducing temporal magnetic field fluctuations in powered magnets would be desirable.